#include <bits/stdc++.h>
using namespace std;

int main()
{
	string S;
	cin >> S;

	int ans = 0;
	int bnum = 0;
	for( int i = 0; i < S.size(); i++ ) {
		if( S[i] == 'B' ) bnum++;
		if( S[i] == 'A' ) ans += bnum;
	}

	cout << ans << endl;
}